Natural gas has increased in significance in recent times on account of its properties as an alternative fuel. From the combustion engine point of view natural gas is interesting above all on account of its high octane content and the consequent high degree of anti-knocking properties and relatively high energy density. A further advantage is the low content of catalytic converter pollutants such as phosphorus and sulfur and the low proportion of carbon in comparison with gasoline or diesel. Natural gas, thus, has good combustion properties with a low degree of emissions of pollutants and lowered CO2 emissions.
Natural gas in a highly compressed form with the designation CNG (Compressed Natural Gas) is used. The compressed gas is filled in the tank under pressure of approx. 200 bar and in the process reduced to one two-hundredths of its volume.
Natural gas fueled vehicles are being offered at present in two different forms, with a bivalent or monovalent internal combustion engine. Bivalent internal combustion engines can be run both with natural gas as well as with gasoline. These however do not use the power of natural gas and make natural gas propulsion more expensive as two complete fuelling systems are required.
Monovalent vehicles are run by natural gas alone or have an emergency tank with up to 15 liters of gasoline. The internal combustion engine is optimized for usage with natural gas propulsion.
Natural gas fueled internal combustion engines were conceived in the past above all with respect to the factor of pollutant emissions. In order to provide an optimal conversion of pollutants in the catalytic converter in dynamic operation as well, a precise stoichiometric combination of the natural gas-air-mixture (λ=1) is necessary. The gas added mixture is therefore successful in certain cases of application with electronically regulated gas injection, for example, by means of a lambda regulation by means of an oxygen sensor and “multi-point” injection in front of the input valve of each of the cylinders of the internal combustion engine.
A crucial distinguishing characteristic from gasoline fueled internal combustion engines is the injection of the fuel in the form of a gas in the intake manifold. In this case a significant proportion of the suction air is displaced by the natural gas. Through this process the charging and, with it, the performance of the internal combustion engine also reduces. It is well known that the performance of a usual reciprocating piston engine, if run with natural gas, is approx. 15% under that of one run with gasoline and under that of charged diesel engines.
A partial offsetting of the minimal performance is, as disclosed in DE 100 62 391 A1, possible through charging the internal combustion engine with natural gas.
A further reduction of the existing performance and consumption disadvantages of natural gas propelled internal combustion engines as opposed to the charged diesel engines of today is possible through the use of lean-burn engine concepts. The lower degree of load on components in the case of lean-burn engines allows a significant increase in the supercharging pressures. However the combustion speed is lowered in the case of very lean mixtures so that an intensified charging motion is necessary as a compensatory measure. Corresponding lean-burn engines comprise along with combustion methods with increased lean-burn running capability an exhaust turbo charging with charge-air cooling. The density proportion is, depending on the concept, between 1:11 and 1:13.
Alongside the developments in the branch of lean-burn engines, there are solution concepts to increase the degree of effectiveness of internal combustion engines fueled by natural gas by the direct injection of the fuel in the combustion chamber. The fuel in the form of a gas is injected in this case at around the end of the compression process under high pressures of 200 bar directly into the combustion chamber located in the piston, of a highly compressed engine without a throttle. Through this a non-homogenous gas-air mixture is formed similar to the case of a diesel engine. Such an internal combustion engine is for example well known from the U.S. Pat. No. 5,329,908 A. When the gas pressure sinks below a pre-determined value however the commencement of the injection process and the length of the injection process has to be adjusted in order to ensure the inflow of the gas. In such a case a homogenous mixture is formed which requires a significantly reduced compression proportion. Since the internal combustion engine is propelled with a constant compression proportion, the compression proportion to be selected is dependent on the type of propulsion with the highest tendency to knocking, i.e. with a homogenous formation of the mixture. As a result of this the running of the internal combustion engine is unreliable and not without problems, in particular under unsteady running conditions, and the degree of effectiveness of the internal combustion engine is not optimal at full load.